Unless you have difficulty with test taking, after 1.5 years of EM this test will be little more than a formality.
Just look at the CD to get a feel for the cases section of the exam. If you have done the bulk of your off-service rotations (Peds, OB/GYN, ortho, etc.) you should have NO PROBLEM passing Step 3.
I would recommend taking it no later than the 2nd half of 2nd year (or 3rd year if you're in a 4 year program) so that you can move ahead with licensing / credentailling in order to get things done in time for your start date as an attending.
Of course, if you have trouble with standardized tests, or if you just squeaked by on Steps 1/2, then you should probably study a bit.
